Quantum lost and found

Abstract

We consider the problem of correcting the errors incurred from sending classical or quantum information through a noisy quantum environment by schemes using classical information obtained from a measurement on the environment. We give conditions for quantum or classical information (prepared in a specified input basis B) to be corrigible based on a measurement M. Based on these criteria we give examples of noisy channels such that (1) no information can be corrected by such a scheme, (2) for some basis B there is a correcting measurement M, (3) for all bases B there is an M and (4) there is a measurement M which allows perfect correction for all bases B. The last case is equivalent to the possibility of correcting quantum information, and turns out to be equivalent to the channel allowing a representation as a convex combination of isometric channels. Such channels are doubly stochastic but not conversely.     

光纤作为量子信道的性能

本文研究光纤作为量子信道在密波分复用时模式间非线性干扰造成的信道噪声的性质 ,和光纤量子信道在该噪声下的经典容量 .用位移算符方法计算量子噪声的累加 ,同时建立了有衰减和噪声时的信道的量子模型 .本文论证如果干扰是各其他模式独立而均等的贡献的叠加 ,信道噪声是热辐射噪声类型的 ,并求出了在给定参数的情况下的经典容量     

一般费米量子信道的经典容量

文章在费米子平均占据数受限的情况下 ,利用费米型量子高斯加性噪声的球谐函数展开方法 ,给出单模费米子在一般费米量子加性信道上传输经典信息的经典容量     

Unified approach to the capacity evaluation of the relay channel

The authors indicate the dependence between the inputs of the relay channel with one auxiliary random variable as Cover, El-Gamal and Salehi have done for the multiple access channel with arbitrarily correlated sources. Then, by considering broadcast and multiple access sub-channels in the relay channel, the authors describe the essential role of the relay with special Markovity conditions on the auxiliary random variable and channel input-outputs, and unify most of known capacity theorems into one capacity theorem. The capacity theorem potentially may be applicable to a more general class of relay channels including at least the relay channels with known capacity.     

Transmission of classical information via quantum entanglement

Abstract

It has been shown that entanglement can improve transmission of classical information in a two-Pauli channel but not in a depolarizing channel. In this letter, we propose a modified channel that can straddle between the two-Pauli channel and the depolarizing channel and analyse the efficiency of transmission of classical states using mutual information. Our modified channel provides a natural mechanism for accounting for the difference in behaviour through these different channels.     

Basic binary state-space model for time-varying communication channels: uncertainty and information capacity

This paper looks at capacity achieving detection strategies for information transfer over time-varying channels. The time-varying binary symmetric channel (TV-BSC) is identified as the basic binary state-space model. Separation of entropies principles and the TV-BSC model-based state-space approach are used to determine the performance bounds for coherent and non-coherent detection over time-varying communication channels. The mutual information rate over the TV-BSC, assuming channel estimation in the presence of channel noise, is shown to be below the channel information capacity because of lack of perfect channel knowledge. Furthermore, it is shown that TV-BSC model-based differential detection has a fundamental advantage over the channel estimation based detection since it theoretically preserves the TV-BSC information capacity when the observation interval approaches infinity. Simulation analysis corroborates the theoretical results, showing that multiple-symbol differential detection practically achieves the TV-BSC capacity in just a few symbol observation times.     

A Novel Quantum Stegonagraphy Based on Brown States

In this paper, a novel quantum steganography protocol based on Brown entangled states is proposed. The new protocol adopts the CNOT operation to achieve the transmission of secret information by the best use of the characteristics of entangled states. Comparing with the previous quantum steganography algorithms, the new protocol focuses on its anti-noise capability for the phase-flip noise, which proved its good security resisting on quantum noise. Furthermore, the covert communication of secret information in the quantum secure direct communication channel would not affect the normal information transmission process due to the new protocol’s good imperceptibility. If the number of Brown states transmitted in carrier protocol is many enough, the imperceptibility of the secret channel can be further enhanced. In aspect of capacity, the new protocol can further expand its capacity by combining with other quantum steganography protocols. Due to that the proposed protocol does not require the participation of the classic channel when it implements the transmission of secret information, any additional information leakage will not be caused for the new algorithm with good security. The detailed theoretical analysis proves that the new protocol can own good performance on imperceptibility, capacity and security.     

Perfect Quantum Teleportation via Bell States

Quantum mechanics shows superiority than classical mechanics in many aspects and quantum entanglement plays an essential role in information processing and some computational tasks such as quantum teleportation (QT). QT was proposed to transmit the unknown states, in which EPR pairs, the entangled states, can be used as quantum channels. In this paper, we present two simple schemes for teleporting a product state of two arbitrary single-particle and an arbitrary two-particle pure entangled state respectively. Alice and Bob have shared an entangle state. Two Bell states are used as quantum channels. Then after Alice measuring her qubits and informing Bob her measurement results, Bob can perfectly reconstruct the original state by performing corresponding unitary operators on his qubits. It shown that a product state of two arbitrary single-particle and an arbitrary two-particle pure entangled state can be teleported perfectly, i.e. the success probabilities of our schemes are both 1.     

Eavesdropping on practical quantum cryptography

Abstract

Practical implementations of quantum cryptography use attenuated laser pulses as the signal source rather than single photons. The channels used to transmit are also lossy. Here we give a simple derivation of two beamsplitting attacks on quantum cryptographic systems using laser pulses, either coherent or mixed states with any mean photon number. We also give a simple derivation of a photon-number splitting attack, the most advanced, both in terms of performance and technology required. We find bounds on the maximum disturbance for a given mean photon number and observed channel transmission efficiency for which a secret key can be distilled. We start by reviewing two incoherent attacks that can be used on single photon quantum cryptographic systems. These results are then adapted to systems that use laser pulses and lossy channels.     

基于海面散射的莱斯MIMO信道容量研究

通过相关莱斯信道模型对海面散射水声多入多出(MIMO)信道容量进行了研究。从Helmholtz-Kirchhoff积分和Fresnel近似出发,得到海面散射信号的空间相关性,由于信道的协方差矩阵不可以表示成发射相关矩阵和接收相关矩阵的Kronecker积,通过Monte Carlo仿真研究了莱斯因子、接收信噪比和空间相关性对MIMO信道容量的影响。从对outage capacity和信道容量的CCDF(Complementary Cumulative Distribution Function)的仿真计算结果可以看出,莱斯衰落的MIMO信道容量并不总是小于瑞利衰落MIMO信道容量,当信道的空间相关性较大和接收信噪比较小时,由于信道的衰落作用将起主要作用,对于较小的收发阵元数,以非衰落的直达信号为主的莱斯MIMO信道容量将大于瑞利衰落的MIMO信道容量。     

Quantum authentication with unitary coding sets

Abstract

A general class of authentication schemes for arbitrary quantum messages is proposed. The class is based on the use of sets of unitary quantum operations in both transmission and reception, and on appending a quantum tag to the quantum message used in transmission. The previous secret between partners required for any authentication is a classical key. We obtain the minimal requirements on the unitary operations that lead to a probability of failure of the scheme less than one. This failure may be caused by someone performing a unitary operation on the message in the channel between the communicating partners, or by a potential forger impersonating the transmitter.     

Nonadditive Transport in Multi‐Channel Single‐Molecule Circuits

As stated in the classic Kirchhoff's circuit laws, the total conductance of two parallel channels in an electronic circuit is the sum of the individual conductance. However, in molecular circuits, the quantum interference (QI) between the individual channels may lead to apparent invalidity of Kirchhoff's laws. Such an effect can be very significant in single‐molecule circuits consisting of partially overlapped multiple transport channels. Herein, an investigation on how the molecular circuit conductance correlates to the individual channels is conducted in the presence of QI. It is found that the conductance of multi‐channel circuit consisting of both constructive and destructive QI is significantly smaller than the addition of individual ones due to the interference between channels. In contrast, the circuit consisting of destructive QI channels exhibits an additive transport. These investigations provide a new cognition of transport mechanism and manipulation of transport in multi‐channel molecular circuits.     

On the capacity of orthogonalised correlated MIMO channels under different adaptive transmission techniques

Orthogonal space-time block codes (STBCs) are known to orthogonalise the multiple-input multiple-output (MIMO) wireless channel, thus reducing the space-time vector detection to a simpler scalar detection problem. The capacity of STBCs over correlated Rayleigh and Ricean flat-fading MIMO channels under different adaptive transmitting techniques is studied. Three adaptive schemes known as optimal power and rate allocation, total channel inversion with fixed rate policy and its truncated variant are studied. Taking into account the effect of channel correlation, closed-form expressions are obtained for the capacity of orthogonalised Rayleigh and Ricean MIMO channels under these adaptive transmission techniques in order to avoid Monte-Carlo simulations     

Communication system model for information rate evaluation of differential detection over time-varying channels

A communication system model for mutual information performance analysis of multiple-symbol differential M-phase shift keying over time-correlated, time-varying flat-fading communication channels is developed. This model is a finite-state Markov (FSM) equivalent channel representing the cascade of the differential encoder, FSM channel model and differential decoder. A state-space approach is used to model channel phase time correlations. The equivalent model falls in a class that facilitates the use of the forward? backward algorithm, enabling the important information theoretic results to be evaluated. Using such a model, one is able to calculate mutual information for differential detection over time-varying fading channels with an essentially finite time set of correlations, including the Clarke fading channel. Using the equivalent channel, it is proved and corroborated by simulations that multiple-symbol differential detection preserves the channel information capacity when the observation interval approaches infinity.     

A quantum mechanical transport approach to simulation of quadruple gate silicon nanowire transistor

In this paper we have used quantum mechanical transport approach to analyse electrical characteristics of silicon nanowire transistor and have compared the results with those obtained using semi classical Boltzmann transport model. The analyse employs a three dimensional simulation of Silicon nanowire transistor based on self consistent solution of Poisson, Schrodinger equations. Quantum mechanical transport model uses the non equilibrium Green's function (NEGF) while the semi classic model doesn't account for tunneling current. The results have shown that Quantum tunneling is significant in inversion condition especially when the channel length is short. For the long devices quantum modeling and semi classical model produce the same result, and tunneling is negligible.     

Instability of a two-phase flow in a rectangular channel

A two-phase (liquid-gas) flow in a short horizontal slit channel of a rectangular cross section with a height of 440 μm has been experimentally studied using a fluorescent method, which allowed the flow pattern to be monitored and its quantitative characteristics to be measured. It is established that two-phase flow regimes in this channel substantially differ from the classical regimes of flow in extended channels of large cross section. It is demonstrated that the formation of various two-phase flow regimes and the transitions between them in short narrow rectangular slits are determined by instability of the liquid-gas flow at the side-walls of the channel.     

Two-dimensional binary halftoned optical intensity channels [optical wireless communications]

The capacity of two-dimensional (2D) optical intensity channels in which transmit images are constrained to be binary-level has been considered. Examples of such links exist in holographic storage, page-oriented memories, optical interconnects, 2D barcodes as well as multiple-input/multiple-output wireless optical links. Data are transmitted by sending a series of time-varying binary-level optical intensity images from transmitter to receiver. Neither strict spatial alignment between transmitter and receiver nor independence among the spatial channels is required. The approach combines spatial discrete multitone modulation developed for spatially frequency selective channels with halftoning to produce a binary-level output image. Data are modulated in spatial frequency domain as dictated by a water pouring spectrum over the optical transfer function as well as channel and quantisation noise. A binary-level output image is produced by exploiting the excess spatial bandwidth available at the transmitter to shape quantisation noise out of band. A general mathematical framework has been presented, in which such systems can be analysed and designed. In a pixelated wireless optical channel application, halftoning achieves 99.8% of the capacity of an equivalent unconstrained continuous amplitude channel using lmegapixel arrays.     

The transmission capacity of a nonlinear classical communication channel

The transmission capacity of a nonlinear classical communications channel that does not obey the well-known Shannon formula describing a linear additive channel is analyzed. According to the Landauer principle, no finite energy is required to transmit a bit of information, which, in principle, makes reversible data transmission possible. The process of data transmission is analyzed using a model of the transmitting medium representing a ferroelectric crystal possessing a center of symmetry. Using this model, it is possible to trace an analogy between the process of bit setting and transmission, on the one hand, and the violation of the lattice symmetry as a result of the second-order phase transition at the Curie temperature, on the other hand. An expression for the transmission capacity is obtained using the well-known cnoidal solution of the corresponding nonlinear equations in the absence of damping. The transmission capacity of an arbitrary nonlinear classical communication channel is estimated.     

单模压缩信道的纠缠辅助容量

用量子特性函数方法得到了纠缠辅助单模压缩信道在限定输入功率下的经典信息容量的表达式,并进行了数值分析.计算结果表明,信道压缩参数越大,其容量也越大.与没有纠缠辅助下的单模压缩信道的信道容量不同,在纠缠辅助下信道容量一般在信源为压缩态时达到,且此时信源与信道的压缩复参数的相角关系相差π.